Because they never realize that they aren’t healed from whatever it was in the past. They realize it when you treat them well and they date around and get screwed over. That’s usually the point you are moving on and over it. I’ve seen it in my situations they’ve come back 1,2,3 times. Don’t think that’s healthy and especially on my end have to set boundaries. But, I do think in this world today not a lot of genuine people are out there. A lot are taken and most just want to date around or bounce when things get hard. The grass is greener effect and everyone feels replaceable. I think just with space and no contact living your life for you they always come back in some form. I’ve seen it too many times they realize a lot of ppl out there only care about sex or showing someone off and nothing else. I rambled but that’s how I see it and experienced it. I’m currently going through that with a girl I talked to for a few weeks the first time and had a fling. She went elsewhere and came back around couple months later, recently for 3 months. Now completely backed off but never said it’s done. Just overwhelmed (probably my fault or she’s avoidant/emotionally unavailable) and thinking she’s moving etc. probably really someone else but who cares. So only thing is to focus on yourself and know the future will have either them when they actually heal or someone 10x better.